Nuclear Stability: What Does Chemistry Have to Do with It?

AJCE, 2020, 10(2) ISSN 2227-5835

NUCLEAR STABILITY: WHAT DOES CHEMISTRY HAVE TO DO WITH IT?

Robson Fernandes de Farias Universidade Federal do Rio Grande do Norte, Cx. Postal 1664, 59078-970, Natal-RN, Brasil. Email: [email protected]

ABSTRACT This paper presents a possibility of approaching the theme "nuclear stability of the elements" that can be used in university courses of basic chemistry, or even in high school. [African Journal of Chemical Education—AJCE 10(2), July 2020]

AJCE, 2020, 10(2) ISSN 2227-5835

INTRODUCTION

As is well known, as far as the constitution of the atomic nucleus is concerned, chemistry is usually focused only in the number of protons, which in turn determines the number of electrons in the electrosphere and therefore the chemical properties of the elements.

However, since chemistry is based on the study of chemical reactions, understands, even in a simplified way, aspects of nuclear stability is interesting (after all, without stable elements we wouldn't even exist, right?).

This paper presents a possibility of approaching the theme "nuclear stability of the elements" that can be used in university courses of basic chemistry, or even in high school.

METHODOLOGY

For each element only the key (most abundant, hence, most stable) isotope was considered.

The atomic data were those available at the Royal Society of Chemistry Periodic Table [1].

For technetium, the radio isotope with large half live was considered [2]. To radon, the isotope 222Rn was chose due to its larger half live (3.8 d) in comparison with 211Rn (4.6 h) and

220Rn (55.6 s) [1]. To uranium it was considered the most abundant (99,3%) isotope: 238U [1]. To americium and curium, the isotopes with larger half live were chose: 243Am and 248Cm [1]. The same reasoning was used for other synthetic elements with more than one isotope. The chosen one was always the one with the longest half live (ie the most stable; the half-live is associated with the radioactive decay mode. Alpha decay was chosen).

The task to be performed by the students can be subdivided into the following steps: a) Search the indicated sites (see references) for the main isotopes for the elements from Z = 1 to

Z = 118; 79

AJCE, 2020, 10(2) ISSN 2227-5835 b) Set up a Table (like Table 1); c) Plot for the 118 elements the number of neutrons in the nucleus as a function of the atomic

number (as in Figure 1); d) Analyzing the Table as well as the graph produced, point out conclusions that can be obtained

regarding the stability of the chemical elements.

RESULTS AND DISCUSSION

The employed data are summarized in Table 1. A plot of the number of neutrons in the nucleus as a function of the atomic number for elements 1-118 is shown in Figure 1.

Table 1. Key isotopes for the known chemical elements (1 to 118). Np (Z) and Nn are, respectively, the number of protons and neutrons in the key isotope.

Element Key isotope Np (Z) Nn Nn/Np hydrogen 1H 1 0 0 Helium 4He 2 2 1 Lithium 7Li 3 4 1.3 Beryllium 9Be 4 5 1.3 Boron 11B 5 6 1.2 Carbon 12C 6 6 1.0 Nitrogen 14N 7 7 1.0 Oxygen 16O 8 8 1.0 Fluorine 19F 9 10 1.1 Neon 20Ne 10 10 1.0 Sodium 23Na 11 12 1.1 Magnesium 24Mg 12 12 1.0 Aluminium 27Al 13 14 1.1 Silicon 28Si 14 14 1.0 Phosphorus 31P 15 16 1.1 Sulfur 32S 16 16 1.0 Chlorine 35Cl 17 18 1.1 Argon 40Ar 18 22 1.2 Potassium 39K 18 21 1.2 Calcium 40Ca 20 20 1.0 Scandium 45Sc 21 24 1,1 80

AJCE, 2020, 10(2) ISSN 2227-5835

Titanium 48Ti 22 26 1.2 Vanadium 51V 23 28 1.2 Chromium 52Cr 24 28 1.2 Manganese 55Mn 25 30 1.2 Iron 56Fe 26 30 1.2 Cobalt 59Co 27 32 1.2 Nickel 58Ni 28 30 1.1 Copper 63Cu 29 34 1.2 Zinc 64Zn 30 34 1.1 Gallium 69Ga 31 38 1.2 Germanium 73Ge 32 41 1.3 Arsenic 75As 33 42 1.3 Selenium 80Se 34 46 1.4 Bromine 79Br 35 44 1.3 Krypton 84Kr 36 48 1.3 Rubidium 85Rb 37 48 1.3 Strontium 86Sr 38 48 1.3 Yttrium 89Y 39 50 1.3 Zirconium 90Zr 40 50 1.3 Niobium 93Nb 41 52 1.3 Molybdenum 95Mo 42 53 1.3 Technetium 98Tc 43 55 1.3 Ruthenium 101Ru 44 57 1.3 Rhodium 103Rh 45 58 1.3 Palladium 106Pd 46 60 1.3 Silver 107Ag 47 60 1.3 Cadmium 114Cd 48 66 1.4 Indium 115In 49 66 1.3 Tin 120Sn 50 70 1.4 Antimony 121Sb 51 70 1.4 Tellurium 130Te 52 78 1.5 Iodine 127I 53 74 1.4 Xenon 132Xe 54 78 1.4 Caesium 133Cs 55 78 1.4 Barium 138Ba 56 82 1.5 Lanthanum 139La 57 82 1.4 Cerium 140Ce 58 82 1.4 Praseodymium 141Pr 59 82 1.4 Neodymium 142Nd 60 82 1.4 Promethium 145Pm 61 84 1.4 Samarium 152Sm 62 90 1.5 Europium 153Eu 63 90 1.4 Gadolinium 158Gd 64 94 1.5

AJCE, 2020, 10(2) ISSN 2227-5835

Terbium 159Tb 65 94 1.4 Dysprosium 164Dy 66 98 1.5 Holmium 165Ho 67 98 1.5 Erbium 166Er 68 98 1.4 Thulium 169Th 69 100 1.4 Ytterbium 172Yb 70 102 1.5 Lutetium 175Lu 71 104 1.5 Hafnium 177Hf 72 105 1.5 Tantalum 180Ta 73 107 1.5 Tungsten 182W 74 108 1.5 Rhenium 187Re 75 112 1.5 Osmium 192Os 76 116 1.5 Iridium 193Ir 77 116 1.5 Platinum 195Pt 78 117 1.5 Gold 197Au 79 118 1.5 Mercury 202Hg 80 122 1.5 Thallium 205Tl 81 124 1.6 Lead 208Pb 82 126 1.5 Bismuth 209Bi 83 126 1.5 Polonium 209Po 84 125 1.5 Astatine 210At 85 125 1.5 Radon 222Rn 86 136 1.6 Francium 223Fr 87 136 1.6 Radium 226Ra 88 138 1.6 Actinium 227Ac 89 138 1.6 Thorium 230Th 90 140 1.6 Protactinium 231Pa 91 140 1.5 Uranium 238U 92 146 1.6 Neptunium 237Np 93 144 1.5 Plutonium 238Pu 94 144 1.5 Americium 243Am 95 148 1.6 Curium 248Cm 96 152 1.6 Berkelium 247Bk 97 150 1.5 Californium 251Cf 98 153 1.6 Einsteinium 252Es 99 153 1.5 Fermium 257Fm 100 157 1.6 Mendelevium 258Md 101 157 1.6 Nobelium 259No 102 157 1.5 Lawrencium 262Lr 103 159 1.5 Rutherfordium 265Rf 104 161 1.5 Dubnium 268Db 105 163 1.6 Seaborgium 271Sg 106 165 1.6 Bohrium 272Bh 107 165 1.5

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Hassium 270Hs 108 162 1.5 Meitnerium 276Mt 109 167 1.5 Darmstadtium 281Ds 110 171 1.6 Roentgenium 280Rg 111 169 1.5 Copernicium 285Cn 112 173 1.5 Nihonium 286Nh 113 173 1.5 Flerovium 289Fl 114 175 1.5 Moscovium 289Mc 115 174 1.5 Livermorium 293Lv 116 177 1.5 Tennessine 294Ts 117 177 1.5 Oganesson 294Og 118 176 1.5

200

180

160

140

120

100

N 80

-20 0 20 40 60 80 100 120 Z

Figure 1. Number of neutrons in the nucleus (Nn) as a function of Z, for elements 1-118.

AJCE, 2020, 10(2) ISSN 2227-5835

CONCLUSIONS

Based on both (Table and graph), some conclusions (some here, as examples, but, of course, the teachers and students could obtain another ones) can be pointed out:

1. The theoretical interpretation that the function of neutrons is to stabilize the atomic nucleus,

acting as a "glue" that compensates for repulsion between protons makes sense, since the

only element that has no neutrons in its nucleus is the one that has only one proton (H);

However, we must remember that hydrogen has two more isotopes: 2H and 3H with one

and two neutrons respectively;

2. For all elements (with the obvious exception of hydrogen), the number of neutrons is equal

to or greater than the number of protons;

3. The ratio (number of neutrons) / (number of protons) slowly increases over the periodic

table, from 1.0 to 1.6. This shows that it becomes progressively harder to "compensate" for

interprotonic repulsion as the element becomes heavier;

4. The curve obtained (Figure 1) shows that the relationship between the increase of the

number of protons and the number of neutrons is linear (r = 0.9980), illustrating the

importance of neutrons to nuclear stability.

REFERENCES 1. https://www.rsc.org/periodic-table (consulted in December, 19, 2019). 2. https://www.webelements.com/technetium/isotopes.html (consulted in December, 19, 2019).